Process for the prevention of frost heaves in fine-grained soils

ABSTRACT

Process for the prevention of frost heaves in fine-grained soils comprising the insertion of a large-pored filter layer which is protected on its upper and lower sides by a permeable, rot-proof web of fabric, felt or fleece.

United States Patent [1 1 Roth Dec. 16, 1975 21 Appl. NO.Z 478,163

[30] Foreign Application Priority Data June 8, i973 Austria 5054/73 [52] US. Cl. 61/36 A; 61/50 [51] Int. Cl. EOZD 3/12; E02D 5/00 [58] Field of Search 61/36 A, 35; l66/DIG. l;

[56] References Cited UNITED STATES PATENTS 1,747,125 2 1930 Murphy 404/27 2,402,597 6/1946 Bourcier 404/27 3,670.506 6/1972 Gavdard 61/35 3,818,712 6/1974 Burt et al. 6l/36 A FOREIGN PATENTS OR APPLICATIONS 4,539,310 l0/l970 Japan 404/72 Primary ExaminerPaul R. Gilliam Assistant ExaminerV. N. Sakran Attorney, Agent, or Firm-Wenderoth, Lind & Ponack [57] ABSTRACT Process for the prevention of frost heaves in finegrained soils comprising the insertion of a large-pored filter layer which is protected on its upper and lower sides by a permeable, rot-proof web of fabric, felt or fleece.

10 Claims, 2 Drawing Figures IA". p, '4 v US. Patent Dec. 16, 1975 I 3,925,993

B IGE PROCESS FOR THE PREVENTION OF FROST I-IEAVES IN FINE-GRAINED SOILS This invention relates to a process for the prevention of frost heaves in fine-grained soils, and in particular, the prevention of frost fractures in roads.

It is known that soils, particularly those which contain more than three per cent by weight of grains with a diameter of up to 0.02 mm, have a tendency to frost heaves at temperatures below C. The water between the grains freezes, and the ice droplets thus formed result in a reduced pressure in the adjacent capillaries, filled with water, of the lower, as yet unfrozen soil layers; this reduced pressure has the effect of a vigorous infiltration of water towards the ice droplets. Thus, the ice droplets rapidly grow in size which finally results in frost heaves and, in the thawing period, in a softening of the sub-soil as a consequence of the accumulation of water.

The most effective precaution against frost damage was hitherto considered to be a replacement of the soil endangered by frost, up to the expected depth of penetration, by frost-resistant gravel. This requires a large material input of high-grade gravel.

Furthermore, types of construction are known which make the dimensions of the base and/or the carriageway surfacing so strong that they are not destroyed either by frost heaves or by a softening of the sub-soil under traffic loads. This process also becomes very expensive due to the increased material costs, e.g. for reinforced concrete carriageway surfacing or soil stabilisation.

It has also been tried successfully to protect sub-soil, endangered by frost, against a cooling to below 0C by heat insulation. Due to the impeded heat exchange between the carriageway surfacing and the subsoil, however, black ice forms on such roads substantially faster than on uninsulated carriageways.

Finally, a process is also known which prevents the water infiltration to the ice boundary, quoted above, by sealing webs below the frost boundary. In order to prevent an accumulation of seepage water in the soil layers sealed in this manner, the expensive sealing must also be carried out laterally and above. Particularly the upper sealing, which lies near to the surface of the carriageway, is thereby subjected to large loads (mechanical load by traffic and embrittlement by frost temperatures).

The present invention involves inserting, for the purpose of interrupting the transport of water in the soil capillaries, in the region of the frost boundary, a largepored filter layer and locating, on both the upper and lower sides of the latter, a permeable, rot-proof web of fabric, felt or fleece in order to prevent contamination of the filter layer by fine soil particles from the adjacent soil layers. It becomes possible thereby to construct filter layers which destroy capillary action, from a thickness of as little as cm, without the danger being present of contamination by the adjacent soil. High-grade material is saved by the insertion of such thin filter layers, because the soil below and above the thin filter layer must indeed be load-bearing, but not frost-resistant.- Accordingly, the present invention provides a process for theprevention of frost heaves in fine-grained soils, in particular of frost fractures in roads, which comprises interrupting the transport'of water in the soil capillaries in the region of the frost boundary by inserting, in the region of the frost boundary, a large-pored filter layer which is protected, on its upper and lower sides, against contamination by fine soil particles from the upper and lower adjacent soil layers by means of a permeable, rot-proof web of fabric, felt or fleece.

The filter layer may consist of loosely introduced grains which are rot-proofed and inert, such as gravel, slag or particles of plastics material, the inter-spaces between the grains having to be sufficiently large so that any seepage water present is not held in the filter layer by capillary action. This capillary action is, for example, explained in a book by T. W. Lambe & R. V. Whitman, M.I.T. Soil Mechanics 1969, page 245, 246. Accordingly, a height of capillary saturation of 6 cm. is indicated for material of a grain size of d 0.82

The effect according to the invention is, however, achieved only if no capillary is filled with water over the entire height of the filter layer. For example, this is achieved if a layer with a thickness of at least 10 cm is inserted with a grain size of 4 mm. If the grain size is smaller, the layer thickness must be increased correspondingly, equally the layer thickness could be reduced with a larger grain size.

The filter layer used in the process according to the invention also may comprise a solid, porous body with appropriate pore dimensions, a mat form or plate form being contemplated in particular. Here too, care must be taken that the thickness and the pore size are so adjusted that it is not possible for any pore to be filled with water, by reason of capillary action, throughout the entire layer.

The depth at which the filter layer is inserted depends upon the depth of penetration of frost, which may be expected, into the soil material in situ or incorporated above the filter layer. In principle it should be aimed for the filter to lie below the 0C isotherm; however, with good technical strength properties of the base located above, it is also possible that a lowering of the frost boundary below the filter layer will not cause any damage. The ice droplets then forming below the filter layer are kept within limits by the high applied load of base and carriageway surfacing. Furthermore, the entire structure above the filter layer acts as a balancing layer for local heavings in the frost period as well as for traffic loads in the thawing period, during which the ice droplets under the filter melt. For this case, protection of the filter layer by a fine-pored, permeable web of fabric, felt or fleece is particularly important, because the softened soil would otherwise clog the filter pores.

If longitudinal drains are present at the sides of the road, the filter layer should be sloping like a roof, for better drainage, and be connected to the drains. If there is no longitudinal drain, the filter layer may be raised up at the sides, in order to avoid edge disturbances, care having to be taken that the infiltration of surface waters is prevented by sealing the top of the raised up edges. Edge disturbances also may be prevented by a lateral sealing of the soil layer, lying above the filter, with sealing webs. The protection, according to the invention, of the roads against frost damage may be achieved if the filter layer is at least 10 cm. thick and, in the case of a loose filling, comprises grains with a minimum grain size of only 4 mm. For practical purposes, it is convenient to use filter layers having a thickness from 15-25 cm, and a grain size ranging from 4-50 mm. Suitable permeable, rot-proof webs are among all those consisting of thermoplastic synthetic resins such as polyolefins, polyvinyl chloride orpolyesters.

Because of their inert character, polyolefines are particularly preferred, particularly polypropylene, polyethylene and copolymers of propylene and ethylene.

In principle, all webs of fabric or fleece, per example fleeces made of staple fibre or continuous filament fleeces, of the materials mentioned, may be used to carry out the process according to the invention, but protective webs of spun'fleeces of continuous filaments are particularly favourable, because they have the advantage of a high tensile strength and do not require any chemical binding at all. It is sufficient if they are, for example, merely consolidated by needlepunching. This fleece may have a weight per unit area of 100-500 g/m preferably of 250500 g/m The invention is illustrated schematically in the accompanying drawing which illustrates examples of two embodiments. FIG. 1 shows the location of a filter layer below the C isotherm to be expected, and FIG. 2 represents the case when the 0C isotherm is below the filter layer.

In the embodiment illustrated in FIG. 1, 1 represents the carriageway surfacing which may be constructed in the customary way. 2 represents the soil above the filter layer which should be load-bearing, but need not be frost-resistant, and thus may have a cohesive character. If the 0C isotherm is above the filter, then water infiltration from the lower soil layer 7 is not possible, since the pores of the filter layer are so large that they cannot be filled with water by capillary action and thus cannot act as a bridge for water infiltration between the upper soil layer 2 and the lower soil layer 7. The upper web of fabric, felt or fleece 4 prevents the coarse filter grains from pressing intothe upper soil layer 2 and pre vents fine soil particles from this layer from penetrating the filter layer, by means of seepage water or otherwise. Thelower web of fabric, felt of fleece 6 serves the purpose of protecting the filter layer 5 against being pressed into the lower soil layer 7. In addition, this layer 6 prevents a pumping-up of softened soil 7, in case the 0C isotherm 3 should sink below filter (see FIG. 2). FIG. 2 illustrates the situation when the 0C isotherm 3 sinks into the lower soil layer 7. Whilst the upper soil layer 2 was unable, because the paths of water infiltration are interrupted by the filter layer 5, to take up any additional water from the lower soil layer 7, as the ice boundary migrated through, water infiltration is now possible. However, the formation of ice droplets in the soil layer 7 is hindered by the applied load of carriageway surfacing 1, upper soil layer 2 and filter layer 5 which are located above. Furthermore, the upper layers l, 2 and 5 act as a balancing layer for more promounced local heavings if they occur. In the thawing period, the upper reach of the lower soil layer 7 has an increased water content, due to the melting of the ice droplets formed there, and thus a lower load-bearing capacity. However, due to the load-distributing action of the layers 1, 2 and 5, fracture of the carriageway surfacing l by traffic loads in the thawing period is prevented. At this time, the lower web of fabric,felt or fleece 6 protects the filter layer against the penetration or pumping up" of softened soil material 7.

The following Example illustrates the process of the invention.

EXAMPLE on a loamy soil with a high water content. A needlepunched fleece of polypropylene continuous filament, having a weight per unit area of 400 g/m and a tear strength of 16 kp/cm was inserted, and this fleece was covered with a layer, 20 cm deep, of round road stone with a grain size of IS to mm diameter. The road stone layer was covered over with a polypropylene fleece of the same properties as the fleece under the round road stone. Filling-in with 40 cm of compactable material followed, and a 6 cm bitumen surface was applied.

The road, which was a district road frequently used, showed no frost damage of any king after one winter.

What we claim is:

l. A process for the preparation of frost heaves in fine-grained soils, in particular of frost fractures in roads which comprises interrupting the transport of water in the soil capillaries in the region of the frost boundary by inserting, in the region of the frost boundary, a large-pored filter layer of loosely introduced grains made of rot-proof inert material, the pores of said filter layer are of a dimension, that any seepage water present is not held in the filter layer by capillary action and is protected, on its upper and lower sides, against contamination by fine soil particles from the upper and lower adjacent soil layers by means of a water-permeable rot-proof web, selected from the group consisting of fabric and fleece made of a thermoplastic synthetic resin.

2. The process according to claim 1, in which the filter layer of loosely introduced grains is a layer of gravel with a minimum grain size of 4 mm.

3. The process according to claim 1, in which the rotproof web is made of a polyolefine, selected from the group consisting of polypropylene, polyethylene and copolymers of propylene and ethylene.

4. A process according to claim 1, in which the rotproof web is a polypropylene continuous filament fleece consolidated by needle punching and having a weight per unit area of -500 glm 5. The process as claimed in claim 1, in which the upper adjacent soil layer is laterally sealed with sealing webs in order to prevent absorption of water from the soil layers adjacent to the road construction.

6. The process according to claim 1, in which the filter layer is a gravel layer with a thickness of at least 10 cm, the grain size of the gravels being 8 to 50 mm.

7. A process for the prevention of frost heaves in fine-grained soils, in particular of frost fractures in roads which comprises interrupting the transport of water in the soil capillaries in the region of the frost boundary by inserting, in the region of the frost boundary, a large-pored filter layer consisting of ,a solid porous body in the form of a plate made of rot-proof inert material, the pores of said filter layer are of a dimension, that any seepage water present is not held in the filter layer by capillary action and is protected, on its upper and lower sides, against contamination by fine soil particles from the upper and lower adjacent soil layers by means of a water-permeable rot-proof web, selected from the group consisting of fabric and fleece made of a thermoplastic synthetic resin.

8. The process according to claim 7, in which the rotproof web is made of a polyolefine, selected from the 3,925,993 6 group Consisting of P yp y p y y and 10. The process according to claim 7, in which the copolymers of propylene and ethylene upper adjacent soil layer is laterally sealed with sealing 9. The rocess accordin to claim 7 in which the rotproof is a polyprogylene continuous filament webs m order to prevent absorptron of water from the fleece consolidated by needle punching and having a 5 layers adjacent to the road constructionweight per unit area of l00-500 g/m 

1. A process for the preparation of frost heaves in fine-grained soils, in particular of frost fractures in roads which comprises interrupting the transport of water in the soil capillaries in the region of the frost boundary by inserting, in the region of the frost boundary, a large-pored filter layer of loosely introduced grains made of rot-proof inert material, the pores of said filter layer are of a dimension, that any seepage water present is not held in the filter layer by capillary action and is protected, on its upper and lower sides, against contamination by fine soil particles from the upper and lower adjacent soil layers by means of a water-permeable rot-proof web, selected from the group consisting of fabric and fleece made of a thermoplastic synthetic resin.
 2. The process according to claim 1, in which the filter layer of loosely introduced grains is a layer of gravel with a minimum grain size of 4 mm.
 3. The process according to claim 1, in which the rot-proof web is made of a polyolefine, selected from the group consisting of polypropylene, polyethylene and copolymers of propylene and ethylene.
 4. A process according to claim 1, in which the rot-proof web is a polypropylene continuous filament fleece consolidated by needle punching and having a weight per unit area of 100-500 g/m2.
 5. The process as claimed in claim 1, in which the upper adjacent soil layer is laterally sealed with sealing webs in order to prevent absorption of water from the soil layers adjacent to the road construction.
 6. The process according to claim 1, in which the filter layer is a gravel layer with a thickness of at least 10 cm, the grain size of the gravels being 8 to 50 mm.
 7. A process for the prevention of frost heaves in fine-grained soils, in particular of frost fractures in roads which comprises interrupting the transport of water in the soil capillaries in the region of the frost boundary by inserting, in the region of the frost boundary, a large-pored filter layer consisting of a solid porous body in the form of a plate made of rot-proof inert material, the pores of said filter layer are of a dimension, that any seepage water present is not held in the filter layer by capillary action and is protected, on its upper and lower sides, against contamination by fine soil particles from the upper and lower adjacent soil layers by means of a water-permeable rot-proof web, selected from the group consisting of fabric and fleece made of a thermoplastic synthetic resin.
 8. The process according to claim 7, in which the rot-proof web is made of a polyolefine, selected from the group consisting of polypropylene, polyethylene and copolymers of propylene and ethylene.
 9. The process according to claim 7, in which the rot-proof web is a polypropylene continuous filament fleece consolidated by needle punching and having a weight per unit area of 100-500 g/m2.
 10. The process according to claim 7, in which the upper adjacent soil layer is laterally sealed with sealing webs in order to prevent absorption of water from the soil layers adjacent to the road construction. 